1. Field of the Invention
This Invention relates to radiation-sensitive elements for colored image reproduction and more particularly to elements wherein positive images are formed on paper substrates for the purposes of proofing the color separations of original, multi-colored, renditions of graphic art.
2. Description of the Prior Art
In the photomechanical process of reproducing the tone values and color shades represented by a piece of original graphic art, it is desirable to prepare a series of color separations so that the particular colors to be printed are first recorded on a set of separation negatives or positives by means of appropriate filtering and masking techniques. These separations were originally done by photographic film, but can now be created by various imaging technologies and even electronic data storage separations are used.
Traditionally the color separations are formed by following known imaging procedures. For example, four photographic black and white separation negatives or positives are made representing the three process colors, magenta, cyan and yellow as well as a fourth representing the black contribution. These separations are then used to make four printing plates. These plates are then mounted on an offset printing press to recreate a rendition of the original piece of art.
In some instances, the printing press is not faithful in its reproduction of the copy carried by the printing plates because of inherent limitations in the printing process. In other instances, the buyer of the printed copy may desire changes in tonal values so that the printed matter as represented by the output of the press is more satisfying to his customers. For these reasons and others, a method of previewing the results before actually printing the subject matter has been devised whereby the changes and adjustments may be made to the separation transparencies prior to the manufacture of the printing plates.
One method of previewing the color separation of an original colored piece of art work is disclosed in U.S. Pat. No. 3,136,637. This patent discloses the use of a multiple layered negative acting proofing system. In this system each of the colored sheets which have been exposed with the appropriate separation negative are developed and then superimposed one on top of the other in exact register. These transparent sheets containing the colored images in exact detail can then be examined individually or in total to recognize any deficiencies compared to the original piece of graphic art prior to the manufacture of the printing plates.
In U.S. Pat. No. 3,671,236, the same objective as above is attained by superimposing on a single substrate the several colored layers so that the composite structure more closely resembles the final printed specimen. This process is again a negative acting system wherein a lithographic negative is the means whereby the information is transferred photomechanically to the colored proofing sheets.
Still another proofing system is disclosed in U.S. Pat. No. 2,993,788 wherein a lithographic negative is used to expose a positive acting diazo oxide sensitized substrate. By exposing, developing and the application of a dye of predetermined color using the non-exposed portions of the diazo oxide as a photomask, a single sheet, negative acting proofing composition is attained.
A particular problem which often occurs in the preparation of photomechanical proofs is that special receptor layers may be needed for accepting the highest quality proofing systems. Because special receiving layers are needed, the image can not easily duplicate the image printed on paper stock or specialty papers because the background of the receptor layer does not duplicate the background color and tone of the paper layer on which the printing is to be performed.
In some systems, such as those described in U.S. Pat. Nos. 5,240,810; 5,192,630; and 5,094,931 require special adhesive layers that are separately laminated or coated onto paper stock. In addition these systems require multiple transfer of the image between receptors. Different imaging systems may have to be used for conventional receptor sheets and systems using paper as the receptor. The use of additional adhesive layer on the receptors also can increase dot gain. The transfer process also limits the lower weight limit of the paper upon which the image may be transferred. Lower weight papers have a tendency to tear, wrinkle or curl. Although this feature can be desirable in certain circumstances, it is also undesirable at many times.
U.S. Pat. No. 4,260,673 describes a presensitized color proofing sheet comprising a carrier sheet having a smooth release surface. Coated onto the release surface of the carrier sheet in a clinging engagement is a color coating of a mixture of a diazo oxide and a pigmented base soluble resin compound. By clinging engagement it is meant that the release value is between 2-60 grains per 2 lineal inches, with between 4 to 20 being preferred, as tested by the procedures described in TAPPI (Technical Association for the Pulp and Paper Industry), vol. 43, No. 8, pp. 164A and 165A (August 1960).
Bonded to the top of this color coating is a discrete barrier layer of a mixture of a base soluble resin and a diazo oxide. Both the barrier layer and the color coating are solubilizable in an aqueous alkali developing medium upon exposure to actinic radiation but not solubilizable in the developing medium prior to exposure to actinic radiation. Firmly attached to the barrier is a clear adhesive layer which is insolubilizable in the development medium. This system, although commercially effective on special receptor sheets, does not offer the flexibility of use on both special receptors and conventional or unconventional paper stock.